This disclosure relates generally to a method and system for regenerating and/or desulfating NOx adsorbers and/or regenerating particulate filters.
In general, diesel engines generally emit less nitrogen oxides (NOx) than a gasoline engine under most conditions, but because diesel engines mostly or exclusively operate on a high air to fuel ratio, the chemistry of the exhaust gas does not favor NOx reduction, because of the excess of oxidizing species. Thus, the reduction of nitrogen oxides, e.g., nitric oxide (NO), nitrogen dioxide (NO2), and nitrous oxide (N2O), in exhaust gas is a widely addressed problem as a result of environmental concerns and mandated government emissions regulations, particularly in the transportation industry. One proposed solution is the use of a three-way conversion catalyst, which can be employed to treat the exhaust gases. Such three-way conversion catalysts, contain precious metals such as platinum, palladium, and rhodium, and can promote the oxidation of unburned hydrocarbons and carbon monoxide , and the reduction of nitrogen oxides in exhaust gas provided that the engine is operated around a balanced stoichiometry for combustion (also referred to as “combustion stoichiometry”). The balanced combustion stoichiometry is typically at an air to fuel ratio between about 14.4 to about 14.7.
However, fuel economy and global carbon dioxide emission concerns have made engine operation under lean-burn conditions desirable in order to realize a benefit in fuel economy. Under such lean-burn conditions, the air-to-fuel ratio may be greater than the balanced combustion stoichiometry, i.e., greater than about 14.7 and may be between about 19 to about 35. When lean-burn conditions are employed, three-way conversion catalysts are generally efficient in oxidizing the unburned hydrocarbons and carbon monoxide s, but are generally inefficient in the reduction of nitrogen oxides.
One approach for treating nitrogen oxides in exhaust gases is to incorporate a NOx adsorber, also referred to as a “lean-NOx trap,” in the exhaust lines. The NOx adsorber promotes the catalytic oxidation of nitrogen oxides by catalytic metal components effective for such oxidation, such as precious metals. The formation of NO2 is generally followed by the formation of a nitrate when the NO2 is adsorbed onto the catalyst surface. The NO2 is thus “trapped”, i.e., stored, on the catalyst surface in the nitrate form. The system can be periodically operated under fuel-rich combustion to regenerate the NOx adsorber. During this period of fuel-rich combustion, the absence of oxygen and the presence of a reducing agent promote the release and subsequent reduction of the stored nitrogen oxides. However, this period of fuel-rich combustion may also result in a significant fuel penalty.
As previously mentioned, exhaust gas streams can further comprise particulate matter such as carbon-containing particles or soot. A particulate filter, commonly used with compression ignition engines, can be used to prevent the carbon particles or the soot from exiting a tailpipe. The particulate filter may be a stand-alone device separate and distinct from devices employing catalytic elements for removing undesirable NOx gaseous components. Carbon particles can be trapped in the particulate filter and then periodically burned to regenerate the filter.
Regeneration of particulate filters can be accomplished by the use of auxiliary devices such as a burner or other heating element. For example, an air-fuel nozzle and an ignition device can be used and operated, when desired, to heat the exhaust gases and the particulate filter to a combustion temperature of the trapped particulate matter. In this manner, the trapped particulate matter can be burned from the filter surfaces to permit a continuous flow of the exhaust gases. Alternatively, an electric heater can be used to generate the heat to initiate the combustion of the trapped particulates. However, these approaches are limited by their energy efficiency, durability, and cost.